Background: The tumor microenvironment plays an important role in the progression and recurrence of tumors and immunotherapy outcomes. The use of immune checkpoint blockers to improve the overall survival rate of patients with advanced hepatocellular carcinoma has yielded inconsistent outcomes. We examined the tumor microenvironment-related genes for their clinical significance and biological functions in hepatocellular carcinoma. Methods: Bioinformatic analysis was performed to screen the differentially expressed genes and to identify the core gene of the tumor microenvironment in hepatocellular carcinoma. The expression of KIF18B in hepatocellular carcinoma cell lines and tumor samples was determined using western blotting, quantitative real-time polymerase chain reaction, and immunohistochemistry. The malignancy-promoting ability of KIF18B was evaluated using Cell Counting Kit-8, colony formation, cell proliferation, migration and invasion, and xenograft tumor assays. Results: KIF18B was identified as one of the core genes in the hepatocellular carcinoma microenvironment and was significantly associated with infiltrating immune cell subtypes and tumor cell stemness. Upregulation of KIF18B was associated with poor clinicopathological characteristics and poor patient outcomes; its downregulation inhibited the proliferation ability of hepatocellular carcinoma cells, which was consistent with the findings of in vivo experiments. Knockdown of KIF18B inhibited epithelial-mesenchymal transition which reduced the migration and invasion abilities of tumor cells. A pulmonary metastasis model confirmed that the downregulation of KIF18B inhibited hepatocellular carcinoma cell metastasis in vivo. Conclusion: KIF18B could be a useful marker for determining the treatment outcomes of immune checkpoint blockers in the context of hepatocellular carcinoma.